Hydraulic excavators require relatively large volumes of oil for their operation and due to the nature of the equipment and the surroundings in which they are used, the oil becomes contaminated with metal particles within a relatively short time so that frequent replacement of oil is required in order to maintain the equipment. Failure to replace the oil results in rapid deterioration, inefficient operation and, ultimately, replacement and since such equipment is very expensive, it is vitally important to provide for maintaining clean oil in the system at all times and to continuously clean the oil of any metal particles incurred into the system.